1. Field of the Invention
The present invention is directed to a method and apparatus for determining the degree of insertion or the axial position of any and all control rods in a nuclear reactor core by analyzing the output signals of a number of strings of fixed location, neutron or gamma ray sensitive incore detectors.
2. Description of the Related Art
Knowing the position of control rods in a nuclear core of a nuclear reactor is essential to safe operation of a nuclear power plant as well as a legal requirement for continuation of an operating license. Currently control rod position, where control rods include safety rods, grey rods, shim rods and displacer rods, as well as specifically "control" rods, can be determined in two different ways.
The first uses a coil stack that sits on top of the reactor vessel in which the control rod drive shaft moves up and down. A magnetic impedance produced voltage generated by the coil stack is proportional to the length of the rod drive shaft residing in the coil stack, thereby allowing rod position to be inferred through the voltage output of each coil stack. Occasionally the rod position indicated by this system can be in question. Conventionally, the rod position at startup, as indicated by the coil stack indicators, is checked against the step demand counters to verify that the indicators are valid. If a rod position cannot be verified, because, for example, a coil stack is inoperable due to a mechanical or electrical failure, flux mapping can be performed to indicate rod position. If the position of the target rod cannot be verified, the rod position indicator must be considered inoperable. A reactor shutdown is usually necessary if more than one rod is considered inoperable.
The second method of determining rod position determines enthalpy rise deviations in core power distribution using core exit thermocouples and an inlet temperature detector to determine control rod position change relative to a rod reference position as described in U.S. Pat. No. 4,927,594. Because enthalpy deviation changes with respect to rod position, the change in rod position can be determined from the magnitude of the deviation. By adding the change to the reference rod position, the actual rod position can thereby be determined. The accuracy of the rod position determined by a system in accordance with this second method, needs to be enhanced and supplemented to provide the operator with the most reliable indication of rod position possible, so that unnecessary reactor shutdowns are avoided.